无法理解'：new'的工作

Question

if (false = FALSE) then  

    :NEW.last_modified := sysdate;
:new.ssn := null;

Answer 1

我们通常使用 :OLD 引用旧值，使用 :NEW 引用行级触发器中的新值。

:OLD 和 :NEW 的值在数据操作语言语句中可能会有所不同。

向table插入数据时

 :OLD = NULL (since no old record)  
 :NEW = Newly inserted value

更新 table

中的数据时

 :OLD = Value exists in the table before the UPDATE statement executes 
 :NEW = Newly received value to replace the existing value of the record

从 table

中删除数据时

 :OLD = Value exists in the table before the DELETE statement executes 
 :NEW = NULL

例如，假设您要在 table Employee 中插入一条新记录作为 Alex;

 :OLD = NULL (since no old record)  
 :NEW = Alex

现在您要将值 Alex 更新为 John

 :OLD = Alex
 :NEW = John

当您删除这条记录时，

 :OLD = John
 :NEW = NULL (since the record has been deleted)

Unable to understand the working for ':new'